The present invention relates to a method of manufacturing a liquid crystal display element. More particularly, the present invention relates to a method of manufacturing liquid crystal display element, the method comprising steps of forming electrodes respectively on first and second substrates, aligning the first substrate with the second substrate, adjusting a gap between the first substrate and the second substrate so that the first substrate is evenly spaced apart from the second substrate by a prescribed distance, and bonding the first substrate to the second substrate.
Generally, a liquid crystal display element comprises first and second substrates and a liquid crystal layer interposed therebetween. In a manufacturing step of the liquid crystal display element, electrodes, thin film transistors (hereinafter referred to as "TFTs"), color filter, and/or an orientation film are provided respectively on surfaces of first and second substrate, and a photocuring sealing material is provided between peripheral edges of first and second substrates. Light energy for curing the photocuring sealing material is irradiated upon the photocuring sealing material so that the first substrate is bonded to the second substrate. The photocuring sealing material is provided on the periphery of an area serving as a display screen when the liquid crystal display element is completed. Liquid crystal is injected under vacuum from an opening for injecting liquid crystal. The above-mentioned opening is provided in the photocuring sealing material beforehand. Finally, the opening for injecting liquid crystal is closed to complete the liquid crystal display element.
FIG. 3 is a sectional view showing one example of a manufacturing apparatus to be used in a method of manufacturing the conventional liquid crystal display element disclosed in, for example, Japanese Unexamined Patent Publication No. 324823/1992. Referring to FIG. 3, reference numerals 5 and 6 respectively show a first transparent substrate and a second transparent substrate both of which are made of glass and the like. TFT and/or an electrode made of metal are formed on at least one of the surfaces opposite to each other (hereinafter referred to as "opposite surfaces"). The opposite surfaces include a surface of the first transparent substrate and a surface of the second transparent substrate. In the present specification the first transparent substrate 5 is called an upper substrate 5 and the second transparent substrate 6 is called a lower substrate 6. Reference numeral 7 shows photocuring sealing material. Reference numeral 8 shows a spacer for maintaining the gap between a surface of the upper substrate 5 and a surface of the lower substrate 6. Reference numeral 2 shows light energy for curing the photocuring sealing material 7. Reference numeral 12 shows an upper surface table for applying the pressure to the upper substrate 5 and the lower substrate 6. The upper surface table 12 is a part of the apparatus for manufacturing the liquid crystal display element. Light energy 2 can be transmitted through the upper surface table. It is not necessary for the light energy 2 to be irradiated only from an upper portion of the liquid crystal display element, also there is a case in which the light energy 2 is irradiated only from an lower portion of the liquid crystal display element. There is a case, the upper surface table 12 and the lower surface table 13 are opposite in the above-mentioned arrangement so that there could be a case, where optical energy 2 can be transmitted through 13.
In the conventional method of manufacturing the liquid crystal display element, the step of adjusting a gap between the upper substrate and the lower substrate so that the upper substrate is evenly spaced apart from the lower substrate by a prescribed distance and the step of bonding the upper substrate to the lower substrate are performed by interposing the upper substrate 5 and the lower substrate 6 between upper surface table 12 and lower surface table 13, applying a pressure to upper substrate 5 and lower substrate 6 and supplying entire surface of the upper substrate including the electrodes with the optical energy 2 for curing photocuring sealing material 7 as shown in FIG. 3.
In the method of manufacturing the conventional liquid crystal display element, the upper substrate and the lower substrate are entirely raised in temperature with light energy. According to a measurement, when light energy of 4000 mJ/cm.sup.2 is irradiated by using a metal halide lamp for emitting light intensity of 80 mW/cm.sup.2 or more to surface of a low alkali glass substrate generally used as the upper substrate and/or the lower substrate, the surface temperature of the low alkali glass substrate is increased up to about 70.degree. C. Accordingly, curvature and distortion of the substrate are confirmed to be caused by difference in thermal expansion coefficient (or linear expansion coefficient) when upper and lower substrates are oppositely bonded, and material of the upper substrate is different from that of the lower substrate.
In Japanese Unexamined Patent Publication No. 127174/1993, there is disclosed a liquid crystal display element made by using an ultraviolet ray curing resin. In the publication, it is described that each surface temperature of the upper substrate and the lower substrate can be maintained at a room temperature when the ultraviolet ray curing resin is cured by irradiation of ultraviolet ray. Actually, it is extremely difficult that each surface temperature of the upper substrate and the lower substrate is kept at a room temperature when the ultraviolet ray is used.
In Japanese Unexamined Patent Publication No. 112128/1986, there is disclosed a liquid crystal display element made by using an ultraviolet ray curing resin. In the publication, it is described that the surface temperature of the upper substrate and the lower substrate can be maintained below about 50.degree. C. when the ultraviolet ray curing resin is cured by irradiation of the ultraviolet ray. Actually, when the light energy of 4000 mJ/cm.sup.2 necessary for curing the ultraviolet ray curing resin is irradiated, the surface temperature of the upper substrate and the lower substrate are elevated to about 70.degree. C.
Further, in Japanese Unexamined Patent Publication No. 211396/1996 and Japanese Unexamined Patent Publication No. 219932/1986, there is disclosed a liquid crystal display element made by using an ultraviolet ray curing resin. In the publications, it is described that the surface temperature of the upper substrate and the lower substrate can be maintained at approximately room temperature when wind of a room temperature or cooling wind is used while the ultraviolet ray curing resin is cured through irradiation of ultraviolet ray. In reality, the surface temperature and the thermal expansion coefficient in the upper substrate are different from those in the lower substrate even under the cold temperature wind of room temperature or the cooling wind when the light energy of 4000 mJ/cm.sup.2 necessary for curing the ultraviolet ray curing resin are irradiated, so as to cause the curvature and distortion of the upper and lower substrates.
Generally, a substrate having a lower thermal expansion coefficient (such as quartz glass substrate) is used in a case in which a temperature of substrate should be high, such as a case where polycrystalline silicon is used for forming a TFT. The thermal expansion coefficient of the quartz glass substrate is 5.5.times.10.sup.-7 /.degree.C. Generally, a low alkali glass substrate is used as a substrate opposite to a substrate on which a TFT is formed (hereinafter referred to as "TFT substrate") in consideration of cost of the liquid crystal display element. The thermal expansion coefficient of the low alkali glass substrate is 44.times.10.sup.-7 to 46.times.10.sup.-7 /.degree.C. Namely, thermal expansion coefficient of the low alkali glass substrate is about ten times as much as that of the quartz glass substrate.
Furthermore, the quartz glass substrate absorbes lesser light and has a lower thermal expansion coefficient than the low alkali glass substrate because the quartz glass substrate contains lesser impurities compared with the low alkali glass substrate. According to a measurement, when light energy of 4000 mJ/cm.sup.2 is irradiated by a metal halide lamp for emitting the light intensity of 80 mW/cm.sup.2 or more in a surface of the quartz glass, the surface temperature of the quartz glass substrate is increased to about 30.degree. C. In a liquid crystal display element of 3 inches of a diagonal line, which is manufactured by using a quartz glass substrate (temperature measured on a surface of the quartz glass substrate is 30.degree. C.) as a upper substrate, and a low alkali glass substrate (temperature measured on a surface of the low alkali glass substrate is 70.degree. C.) as the lower substrate; there is caused difference in expansion of about 13 .mu.m in a diagonal direction. When the difference in expansion is caused by the expansion due to heat, an aperture ratio of peripheral area of the liquid crystal display element is lowered as compared with the aperture ratio of central area of the liquid crystal display element. It is not preferable that the fluctuation of the aperture ratio is large within the liquid crystal display element because nonuniform of luminance of image is caused. When pixel size is 30 .mu.m.times.30 .mu.m (for example, in a liquid crystal display element serving as a video graphics having a TFT formed by polycrystalline silicone increasing which to high temperature in a forming process (hereinafter referred to simply as "high temperature Poly-Si"), the difference in expansion of 13 .mu.m generated in the diagonal direction allows about 30% in aperture ratio to reduce, and therefore the difference in expansion is a problem in practical use. Hereinafter the above-mentioned video graphics is referred to as "high temperature Poly-SiVGA".
Even when material of an upper substrate is the same as that of a lower substrate opposed to the upper substrate, difference in temperature therebetween is caused when the light energy is irradiated from either of upper side and lower side of the opposing two substrate, and therefore the same problem described above is caused. In order to remove the difference in temperature, the light energy can be irradiated from both sides. In this case, there should not be any substrate that shields light beam on an optical path to the photocuring sealing material. When UV (ultraviolet lays) lamp cooled by a cooling water generally, which is used as an light source, is placed on the upper side in the manufacturing step, a fatal defect is applied to products when cooling water is leaked. Because of such a reason, the light energy is irradiated from either of upper side and lower side of the opposing two substrate, especially from the lower. A substrate on which TFT is provided absorbs light beam because the metallic thin film is formed on the substrate, thereby causing the temperature difference easy. According to a measurement, the temperature difference of 10.degree. C. or more is caused between the opposing upper substrate and lower substrate. When UV sealing material is used as a photocuring sealing material in a large size substrate of 550 mm.times.650 mm, the distance between one end and the other end of each diagonal line in each of upper and lower substrates is 850 mm. If there is temperature difference of 10.degree. C. between the opposing upper substrate and lower substrate, there is a difference in expansion of about 40 .mu.m in the diagonal direction between upper substrate and lower substrate. When the difference in expansion is caused by thermal expansion, the aperture ratio of peripheral area of the liquid crystal display element is lowered as compared with the aperture ratio of central area. It is not preferable that aperture ratio within the liquid crystal display element is widely varied because nonuniform in luminance of image is caused. When there is a difference in expansion of 40 .mu.m between upper substrate and lower substrate the opposing in the diagonal direction thereof, the aperture ratio changes by about 30% to cause a serious problem in practical use, provided that the pixel size is 100 .mu.m.times.300 .mu.m (corresponding to XGA (Extended Graphics Array)) in a liquid crystal display element having a TFT formed by general amorphous silicon (hereinafter referred to simply as "a-SiTFT").
The positional shift between the opposing first substrate and second substrate causes lower lluminance or inferior indication due to the reduction of the aperture ratio of pixel. In a liquid crystal display element driven by active matrix driving method and including an active element such as TFT or MIM (metal-insulator-metal), a plurality of pixels are provided within a displaying area. In each pixel, active element is formed, and in displaying area of lower substrate, there are formed one signal lines for scanning the active element and another signal lines for inputting an image signal into each pixel, the one signal line and the another signal line forming a grid pattern. Each pixel is opposite to electrode which is formed on the lower substrate is electrically connected to the active element. In a color display, upper substrate opposed to the lower substrate is a substrate on which a color filter comprising red areas, green areas and blue areas is formed. Clearance between one area and another area adjoining the one area, namely, over the signal lines, there is a light shielding portion called BM (Black Matrix). When a positional shift is caused between the opposing upper and lower substrates, a lightproof operation in the pixel occurs due to the BM provided over a peripheral edge of the pixel. This is a problem caused by reduction of aperture ratio, so that reduction of lluminance is caused. When the positional shift is caused extremely, the light which is transmitted through a coloring matter comprising one pixel enters into another pixel adjoining the one pixel, and accordingly the positional shift is problem because the positional shift causes inferior indication.